Quick reference
Scan this table first. Details for each row are below.
MAOIs
The only outright contraindication in the Qsymia interactions table.
Monoamine oxidase inhibitors
Avoid or discussTaking phentermine with an MAOI increases the risk of hypertensive crisis, so Qsymia is contraindicated during MAOI treatment.
The label says concomitant use of phentermine with monoamine oxidase inhibitors increases the risk of hypertensive crisis, and that Qsymia is contraindicated during MAOI treatment and within 14 days of stopping an MAOI. Your pharmacist can confirm whether a medicine is an MAOI. If you have taken one recently, say so before Qsymia is prescribed — the 14-day gap is not negotiable in the label.
Alcohol and other CNS depressants
The label does not ban alcohol, but it does ask you to keep it modest.
Alcohol, benzodiazepines, barbiturates and sleep medicines
Use cautionThese may add to the dizziness and cognitive effects of phentermine and topiramate.
The label says the concomitant use of alcohol or CNS depressant drugs — it names barbiturates, benzodiazepines and sleep medications — with phentermine or topiramate may potentiate CNS depression such as dizziness or cognitive adverse reactions. Its instructions are practical: advise patients not to drive or operate machinery until they have enough experience on Qsymia to judge whether it affects their mental performance, motor performance or vision; caution patients against excessive alcohol intake; and consider dose reduction or stopping if cognitive dysfunction persists. The Medication Guide puts it more plainly: do not drink too much alcohol while taking Qsymia.
Oral contraceptives
The label is specific here, and the specific answer is reassuring.
Combination oral contraceptives
Generally okayExposure to the hormones shifts and spotting may be more frequent, but the label does not expect a higher risk of pregnancy.
In a study of Qsymia 15 mg/92 mg taken with a single dose of an oral contraceptive containing 35 micrograms of ethinyl estradiol and 1 mg of norethindrone, exposure to the estrogen fell by 16% and exposure to the progestin rose by 22%. The label says this interaction is not anticipated to increase the risk of pregnancy, but that irregular bleeding — spotting — may occur more often, because the endometrium is stabilised less well. Its instruction is explicit and worth repeating: do not discontinue your combination oral contraceptive if spotting occurs, but tell your healthcare provider if the spotting is troubling you.
Diuretics and potassium
This one comes with a blood test attached.
Non-potassium-sparing diuretics
Use cautionCombining them with Qsymia may potentiate potassium loss, so potassium is measured before and during treatment.
The label says concurrent use of Qsymia with non-potassium-sparing diuretics may potentiate the potassium-wasting action of those diuretics, and that hydrochlorothiazide alone with topiramate alone raised topiramate exposure by 27 to 29%. Its instruction is to measure potassium before and during Qsymia treatment when the two are used together. This sits alongside the label warning that Qsymia can increase the risk of hypokalaemia in its own right, through topiramate inhibiting carbonic anhydrase.
Antiepileptic drugs
Topiramate is itself an epilepsy medicine, so this section carries real weight.
Phenytoin, carbamazepine and valproic acid
Use cautionSome lower topiramate levels substantially; valproic acid combined with topiramate has been linked to hyperammonaemia and hypothermia.
The label reports that in people with epilepsy, phenytoin decreased plasma topiramate concentrations by 48% and carbamazepine by 40%, compared with topiramate given alone. It also says concomitant administration of valproic acid and topiramate has been associated with hyperammonaemia with and without encephalopathy, and with hypothermia with and without hyperammonaemia. Its instruction is to consider measuring blood ammonia in anyone who develops hypothermia or encephalopathy.
Other carbonic anhydrase inhibitors
A class you may not realise you are taking — some glaucoma drops and diuretics belong to it.
Carbonic anhydrase inhibitors
Avoid or discussCombining them with topiramate may worsen metabolic acidosis and raise the risk of kidney stones.
The label says concomitant use of topiramate with any other carbonic anhydrase inhibitor may increase the severity of metabolic acidosis and may also increase the risk of kidney stone formation, and that the combination should be avoided. If it cannot be avoided, the label says to monitor for the appearance or worsening of metabolic acidosis. Related to this, the Medication Guide says to drink plenty of fluids while taking Qsymia to help decrease the chance of kidney stones, and to call your provider about severe side or back pain, or blood in your urine.
Amitriptyline and pioglitazone
Two named medicines with smaller, more specific effects.
What the label says about each
Use cautionAmitriptyline levels can rise sharply in some people; pioglitazone exposure falls.
On amitriptyline, the label says some patients may experience a large increase in amitriptyline concentration in the presence of topiramate, and that any dose adjustment should be made according to clinical response rather than to measured levels. On pioglitazone, it reports a decrease in exposure to pioglitazone and its active metabolites when it is used with topiramate, says the clinical relevance is unknown, and suggests considering increased glycemic monitoring when the two are used together.
Common questions
Written and maintained by one person · Built with agentic AI tools · Every number checked against the cited trial · Not medical advice
- Qsymia (phentermine and topiramate) prescribing information — Drug Interactions (7, Table 5), Warnings and Precautions (5.7, 5.10, 5.11, 5.12), Medication Guide View source ↗
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